Short‐Term Dynamics of Low Molecular Weight Organic Substances and Biochar in Arable Soils

Background Main transformation pathways of low molecular weight organic substances (LMWOS) are understood, but only limited knowledge exists on their transformations in different soils and on their interactions with biochar. Aim Objectives were to study short‐term pathways of 13 C‐labelled LMWOS with different functional groups in the presence and absence of biochar in arable loess soils. Methods Soils from three sites were incubated with or without artificially aged biochar and 13 C‐labelled acetate, alanine or glucose at different rates (10 or 50 µmol C g −1 soil) at 60% water holding capacity and 15°C for 5 days, and total and substrate‐derived CO 2 ‐C and microbial biomass C (MBC) were determined and analyses of variance were calculated. Results Cumulative CO 2 ‐C (ΣCO 2 ‐C) emission was significantly ( p ≤ 0.05) affected by substrate rate and type and their interaction. Biochar significantly stimulated total, but not substrate‐derived ΣCO 2 ‐C (ΣCO 2 ‐C SD ) emission. Box‐Cox transformed MBC was significantly affected by site, substrate rate and type, whereas biochar had no significant effect. Substrate‐derived MBC (MBC SD ) and carbon use efficiency (CUE) were significantly affected by site, substrate rate and type and their interaction. Conclusions MBC SD and CUE results at low addition rate confirmed the greater importance of glucose for the build‐up of MBC compared to acetate and alanine, whereas the latter were mineralized to a greater extent. Biochar, once it is aged, which is the typical biochar form in soil, did not significantly affect build‐up of MBC SD and CUE and only slightly increased total, but not ΣCO 2 ‐C SD emission.